Backup bullet seal with actuation delay feature

A sealing system features a primary and secondary bullet seals where the secondary seal is preferably bidirectional and is activated to spread fins with a t-shaped ring that is driven with differential pressure to advance after temperature exposure softens a stop material on the extending portion of the t-shaped ring. Opposed t-shaped rings are disposed at ends of the backup ring that have fins at both ends. Advancement of the t-shaped ring spreads the fins and retains them in the spread condition for enhanced sealing. Optionally the primary seal can also be actuated with a similar t-shaped ring with a temperature sensitive delay feature.